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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 20 May 2011 by Rob Painting

People living on coral atolls have long been the 'poster child' of global warming, and the threat posed by rising sea levels. Some emotive reporting in the media (and rightly so) tend to give the impression that the atolls should have been under water by now. Clearly this isn't the case, as Webb & Kench (2010) show, the sea level rises experienced in the South Pacific so far (2mm per year) have had little effect on the loss of land area on the atolls. Despite some people being confused regarding the timescale involved, the threat is still very real.

The ocean is, in fact, littered with thousands of former coral reefs and atolls which 'drowned' when they failed to grow fast enough to match sea level. Being generally less than 3 metres above sea level, coral atolls are particularly vulnerable to rising seas. People living on coral atolls in the Pacific will be displaced from their homelands in the coming decades once sea level rises above the solid reef foundations which formed during a temporary regional sea level highstand 4,000 to 2,000 years ago. When this happens, the formerly stable atolls will be subject to erosion by waves, long before the atolls are completely submerged.

Coral atoll formation

Coral reefs occupy tropical and sub-tropical waters and accumulate as generations of coral build upon the "bones of their ancestors". Given the right conditions coral are able grow fast enough to keep pace with moderate rates of sea level rise. So what appears to be solid rock formed by the Earth's geological processes, is in fact the result of biological activity over many years.

In the mid-19th century, after voyaging throughout the Pacific, Charles Darwin demonstrated remarkable insight by putting forth the idea that coral atolls are the remnants of submerged ocean volcanoes (Darwin 1842). He envisioned that, as volcanic activity ceased and the volcano slowly submerged beneath the sea, coral were able to grow on it's shores, keeping pace with the subsidence rate and therefore able to maintain a presence at sea level. At the time however, Darwin was unaware of the huge changes in sea level which accompanied the Earths transition in and out of the Quaternary ice ages, so the process is more complex than he realized.

We now know that as the planet cooled at the onset of each ice age, more and more of the Earth's water became locked up in the growing ice sheets. At the last glacial maximum, sea level fell over 120 metres lower than modern sea level. This left coral high and dry and their remains were exposed to chemical weathering by carbonic acid in a process known as karstification. It is this weathering process which is responsible for the bowl-like shape which many coral reefs take on (Purdy & Winterer 2001).

Core samples drilled deep from coral atolls (Royal Society of London 1904, Ladd & Ingerson 1953) show that Darwin was correct. Atolls consist of old coral reef (limestone) sitting on top of a volcanic rock base. Often the reef limestone is hundreds of metres thick, consisting of layers of reef growth that have built up over many millions of years. During the ice age cycles the atoll tops have been repeatedly submerged and exposed as sea level fluctuated up and down (Purdy 1974). While exposed to the air the atoll tops have been eroded down, and when sea level rose again during the interglacials, new coral reef has reclaimed the atoll summit. The top 10 -15 metres of many atolls represent the "recent" growth during the current interglacial, the Holocene. So rather than a continual process, as Darwin originally proposed, coral atoll formation has been a rather stop-start affair.

Figure 2 - Schematic of atoll history through ice age sea level fluctuation. a) sea level thousands of years before & up to present. b) Atoll form relative to sea level. From Woodroffe 2007

The Darwin Point & Guyots

A Darwin Point defines the threshold at which a coral reef "drowns". It refers to a point where changing environmental conditions means the reef cannot grow fast enough to keep in touch with the sea surface, and therefore receive enough sunlight to grow (Grigg 1982). It therefore effectively drowns and dies. Guyots are flat-topped seamounts deep below the sea surface. Thousands of guyots are spread throughout the Pacific. These were once coral atolls, but "drowned" when they passed the Darwin Point.

Because coral reefs flourish in a narrow range of environmental conditions, coral growth can slow down when waters become too hot, too cool or too acidified (Flood 2001), (Scheibner & Speijer 2008). The Emperor seamounts in the Hawaiian Island chain are a case in point. These volcanic islands sit on a continental plate which has gradually drifted northwest into cooler waters over the timespan of tens of millions of years. As the northernmost atolls passed the Darwin Point they "drowned". This occurred in a sequential fashion as each atoll passed the threshold. Kure atoll, is the next in the chain destined for submersion, as it's coral reef growth has almost dropped to zero (Grigg 2008)

Holocene Reef Growth

Coral reef growth during the Holocene, like that of ancient reefs, also has a complicated history. Many reefs around the world show evidence of back-stepping, a point where the reef is unable to keep up with sea level and drowns, but new coral is able to re-establish in shallower water closer to shore. Many regions also harbour relict reefs which have drowned during the Holocene. One vast relict complex exists in deeper water off the Great Barrier Reef. All of which is suggestive of rapid jumps in sea level rise, probably from the collapse of the large glacial lakes on the gigantic Northern Hemisphere ice sheets as they disintegrated (Blanchon & Shaw 1995). The full picture, yet to be fully resolved, may be more convoluted.

Pacific sea level highstand in the Holocene

At around 8-9 thousands years ago sea level caught up with the tops of the eroded atolls and new Holocene reef growth began to be cemented over the older reef foundations. Sea level reached a highstand in the Pacific between 4-2000 years ago , Pirazzoli 1987, Dickinson 2003, Woodroffe & McLean 1998). There is also evidence of a highstand in the Indian Ocean at the Maldives (Kench 2009). Coral reef tops grew to this higher sea level but were exposed once local sea level fell again, through ocean siphoning (Mitrovica & Milne 2002).

This process meant that, unlike sand cays where debris and sediment is constantly shifting (Flood 1986, Flood & Heatwole 1986) coral atolls had solid foundations above sea level upon which organic matter could accumulate. Excluding major storms these foundations made the atolls resistant to wave damage, and therefore stable over the last few thousand years.

Dating of human artifacts at archeaological sites shows Pacific atolls have not been inhabited for more than 1000 to 1500 years Dickinson 2009, which is consistent with the history of Holocene atoll formation.

The end of atoll nations

Now that global sea level is rising once more, the solid foundations (reef flats) which underpin the stable Pacific atolls will be overtopped by the sea at some point. Many of these are 0.5 to 1 metre above local high tide. Submerging these solid foundations will lead to the atolls being vulnerable to wave damage. Erosion will begin to sweep away the rubble, sediment, and thin soils, making them uninhabitable at some point in the future. Dickinson 2009 has constructed a table of estimated crossover dates for Pacific atolls. These are dates, based on projected rates of sea level rise, where the solid reef foundations are over-topped. Generally these cross-over dates occur mid 21st century at the earliest.

Homeless

As we've seen, coral reefs aren't always capable of keeping pace with sea level rise. To do so they need environmental conditions to be within their 'goldilocks zone'; move outside that and they are liable to drown. It just so happens that global warming is in the throes of doing that very thing. Ocean acidification and coral bleaching will severely curtail or even stop reef growth completely (Silvermann 2009).

But more significantly, a regional sea level highstand thousands of years ago formed the stable foundations upon which the soils and vegetation developed. Even if the atoll coral reefs were able to keep pace with future sea level rise, it won't stop the sea rising above the old reef flats and exposing the atolls to persistent wave attack.

So, although coral atolls may grow as sea level rises, this hasn't always been the case in the past, and won't be the case in the future. Atoll islanders may be hanging in there right now, but eventually global warming, and the rising seas, will make them homeless.

An interesting and informative article, particularly as it relates to the following comments in Ian Plimer's Heaven and Earth. On page 310 "the ocean floor around Tuvalu is sinking" and on page 321, "Tuvalu's problems have nothing to do with global climate change."

#2 Stephen Baines "Rob, when you say a Guyot is 1500 m "below the sea surface", I think you mean "above the seafloor." That would be pretty deep to be a drowned atoll."

A number of references indicate that 1500m below the sea surface is not atypical. For example, Encyclopedia Britannica: "In the Pacific Ocean, where guyots are most abundant, most summits lie 1,000 to 2,000 metres (3,300 to 6,600 feet) below sea level."

Just remember that any seamount can deep below current sea surface because of subsidence, not that sea was once that low. Mid-ocean ridge volcano cones move into deep water as the ocean crust subsides and the plate expansion moves it away from the spreading center. Determining whether something was once an atoll in its dim and distant past would at first look involve looking at morphology.

Yes, that's of course correct. I was thinking in terms of guyots formed during the last few glacial cycles, as that is what the post focuses on. But older guyots could get quite a bit deeper as plates move.

There is a technical definition that a guyot has to be a certain height above the seabed. I thought it was 1km, but that's from memory.

Stephen Baines, re- guyot depth, actually I was certain my original reference was correct, but couldn't find the study where I got that information from (my computer is a jumble of thousands of papers and links).

" Dickinson 2009 has constructed a table of estimated crossover dates for Pacific atolls. These are dates, based on projected rates of sea level rise, where the solid reef foundations are over-topped. Generally these cross-over dates occur mid 21st century at the earliest."

The report forgets to say that these dates are based on an extrapolation of the SLR to 1m in 2100. If it were only 50cm, the cross-over dates would only be after 2100 - it it keeps on rising of course. Also a rarely mentioned fact is that the 1 meter SLR predicted by Rahmstorf et al. implies a quadratic acceleration term, which is only possible with a long timescale response where dL/dt is proportional to T (instead of L prop to T). But this implies also that the current SLR is well below the equilibrium value and will continue whatever we do, for centuries. In other words : if the SLR exceed 50 cm, it means that the long timescale term is large, and there is no hope to stop it before 2050. If not , there is no need to do it .

Another side remark is that the economy and the population of atolls have exponentially risen because of the tourism industry , which have cause a massive arrival of tourists and the associated employees. Stopping the use of fossil fuels , and hence probably suppressing overseas vacations, would probably impact the economy and the population much more cruelly than a SLR at 3mm/yr.

"Stopping the use of fossil fuels , and hence probably suppressing overseas vacations, would probably impact the economy and the population much more cruelly than a SLR at 3mm/yr."

Yes, this is a common Straw-man argument brought up by the Denialist Industry-one which fails on several key facts:

1) No-one is talking about the immediate & complete cessation of fossil fuel use, so why mention it at all? We're talking about a significant reduction in the fossil fuel component of domestic electricity & fuel.

2) Air travel is, currently, one of the *smallest* contributors to total GHG emissions, per capita, so will almost certainly be a lower priority than the GHG emissions from electricity generation & domestic travel.

3) Pilot studies have already shown at least one viable, carbon neutral substitute for aviation gasoline (algae derived bio-fuel). Also, with the first ever successful solar powered flight this year, solar powered planes (of one kind or another) will probably dominate our airways by sometime in the latter half of this century.

4) Another GFC will probably hurt the economy of the Pacific Islands much more heavily than an immediate cessation of all fossil fuel consumption-which merely highlights how we should be doing more to help these economies transition to a more stable source of income-rather than using that instability to justify inaction on climate change.

The first part of your argument is equally weak, as you've clearly ignored the fact that (a) melting of fresh-water ice is *accelerating*-not remaining constant & (b) this is coupled with ever increasing thermal expansion of sea-water, which will accelerate the sea-level rise even further. So in fact the predictions by Dickinson are probably very accurate. Would you be prepared to bet your home, & life, on the prediction being wrong Jarch? Then don't be so quick to bet other people's lives & homes so quickly on such weak assumptions.

Marcus, I doubt that keeping the long distance vacations would be a highest priority if there is a strong pressure to reduce the use of fossil fuels, and that bioalgae (or worse solar powered planes !!) would be able to power all airplane companies, not speaking of allowing a large majority of the world population to access them. Air travel are a minor component only because they are accessible only to a very tiny part of the population - and atoll economy is accordingly a very tiny amount of the world economy. Are you suggesting that the world should do everything to allow a very tiny part of the population to have vacations, to allow an even tinier part of the economy of very small countries to survive ? well if you extend this thought to all kind of activities, I don't see how we would reduce our overall consumption !!

obviously you didn't catch the first argument : I didn't say Dickinson's predictions aren't "accurate" (also they publish two hypothesis differing by a factor two, so I don't know which of them is "accurate" ?) : I said that IF the 1m SLR is real, then mathematics also tell us that it is impossible to stop before 2050, whatever we do, because the phenomena you're describing need several centuries to change.

Jarch, to put it into some context for you. An average sized car generates slightly more CO2/passenger-Kilometers of travel than a Long Haul international flight (assuming the only travel done is at peak hour). Of course, in the space of a single year, most car commuters will travel more kilometers in their car than they will in either domestic or international travel. I think you need a better argument for inaction on fossil fuel consumption.

Again Jarch, a long haul aircraft generates approximately 230g of CO2 per passenger-km of travel. A single, mid-sized car generates approximately 280g of CO2 per passenger-km. Guess how many cars in the world there are Jarch? Guess how many kilometers they travel every year Jarch? Fact is that you could *quadruple* the number of people currently engaged in international flights & it would still not come close to the total CO2 contributions of car travel-let alone electricity consumption. Also, domestic flights have a *larger* Carbon Footprint than International Flights, which just makes your argument even *weaker*. Like I said, if you're going to try & defend the fossil fuel industry, then you're going to need a far stronger argument than "oh, we have to protect the economy of those poor Atoll dwellers", because your basis for that argument is completely flimsy & bogus.

"I said that IF the 1m SLR is real, then mathematics also tell us that it is impossible to stop before 2050, whatever we do, because the phenomena you're describing need several centuries to change."

This is, again, a very very weak argument for taking it slow or-worse still-doing nothing at all. The issue right now is to slow the trajectory of warming, & thus slow the rate of acceleration in current sea-level rises. This *will* have the impact of giving us more time before all of these Atolls are permanently drowned, as well as reduce the length of time we need to wait before the Atolls are above sea-level again. Of course it *would* have been better had we acted sooner-you know, when we first had the evidence, but people such as yourself made sure *that* never happened. Still, no doubt you & Gilles would get along well, given that you so clearly think alike.

Marcus, my argument is that it would be very weird to prevent people from using their car for their all day life and allow them to travel over long distances - most people would probably prefer keeping the former and giving up the latter.
"The issue right now is to slow the trajectory of warming"
the issue is that if the 1m SLR is real, it is impossible to slow it significantly before 2050. You could only change the date by a couple of years, which is much smaller than the uncertainty on what will really happen, and without changing the final issue - so you would have to prepare the evacuation of the threatened atolls in any case.Do you know what a time scale is ?

"my argument is that it would be very weird to prevent people from using their car for their all day life and allow them to travel over long distances - most people would probably prefer keeping the former and giving up the latter."

Which is an incredibly *weak* argument, & will remain so no matter how many times you say it. *All* air travel-both domestic & international-makes up barely 3.5% of *all* CO2 emissions, whereas driving cars contributes about 15% of all CO2 emissions. So you see that giving up the latter will do almost *nothing*, whereas even *reducing* our car use will do so much more. Also, whereas there is no useful substitute for air travel, there are plenty of useful substitutes for individual car travel-car pooling, tele-commuting & public transport-just to name a few. So you see, contrary to your claims, I think that people *will* be more likely to give up their current usage of cars-which will have a much greater impact on climate change-than to give up international plane flights-which would have minimal impact on climate change anyway.
Still, Jarch, thanks for reintroducing everyone here to a typical Denialist Argument-totally illogical & poorly constructed....and based on total fallacies.

"the issue is that if the 1m SLR is real, it is impossible to slow it significantly before 2050."

Well, given the weakness of your other argument, how can *anyone* put even the tiniest amount of trust in this claim. Any slowing of the trajectory of warming & sea-level rise *now* is better than just letting things not only continue at their present rate but, given your preferred
scenario, proceed at an ever accelerating rate. Its true we might have to eventually evacuate those on the threatened atolls but, when they are evacuated, I hope they sue people like you who delayed action on climate change for the past 20 years with your incredibly dopey arguments.
Still, given your obvious troll-like behaviour, I think its pretty obvious that you're just Gilles under a different name. I suspected it earlier, but now I feel its confirmed-& I'd be quite happy for Moderators to delete comments #15-#22, to prevent this topic being completely hijacked yet again.

Marcus, your argument only relies on the fact that air travels are restricted to a very small part of the world population, (the richest one most obviously), whereas cars are used by much more people. If you take the consumption PER CAPITA, considering only those who are actually using them, the use of airplanes is of course much more energy consuming than cars - just because as you notice yourself, the energy used per mile is comparable, but people traveling by air do much longer trips ! so what you're really saying is that you would ask modest people to accept the constraints of public transportation, car pooling, and so on, but let a very small subset of rich people spend freely their vacation in very distant islands ? extremely weird, and politically totally unacceptable.
concerning the rate of SLR, just have a look on the peer-reviewed literature, such as :

and look at figure 4 : what is the difference between the "extremes" B1 (yellow) and A1FI (blue) scenario around 2050 ? what is the difference of the time needed to reach a given level ? some years, not more. That's what I said. The difference is in the uncertainty of the acceleration term, which is a totally natural factor - it is pretty insensitive to the scenario actually. What it really means is that if their is a very long term acceleration factor, we are already much above the equilibrium value needed to limit the SLR before 2050 , and it is much too late to avoid it.

"your argument only relies on the fact that air travels are restricted to a very small part of the world population, (the richest one most obviously), whereas cars are used by much more people. If you take the consumption PER CAPITA, considering only those who are actually using them, the use of airplanes is of course much more energy consuming than cars - just because as you notice yourself, the energy used per mile is comparable, but people traveling by air do much longer trips !"

Your entire argument is completely & utterly *false*. I already pointed out that cars generate *more* CO2 per person-km than a plane flight, that long-haul flights generate fewer emissions per passenger-km than shorter domestic flights (as much of the fuel is used in take off & landing-not in actual flight) & that the number of km driven-by a single driver-per year is *greater* than the average distance traveled by an air traveler over the same time period.
Your claim about air travel being restricted to a rich subset is equally ludicrous. The US alone records as many as 2 million people flying *per day*-& therefore probably around 370 million people flying per year....in America alone. Hardly sounds like a tiny subset of rich people.
Again, though, as I said (but clearly you missed), no-one is telling people to simply *cease* driving-so that's just another straw man. They're simply being asked to make more sensible use of their cars-by car-pooling &/or using public transport for instance-or using cars powered with alternative fuels. So going without a car for day-to-day commuting is a much simpler-& probably more cost effective-prospect than asking those hundreds of millions of people, world wide, currently traveling by plane to find some other means of traveling overseas.
The upshot being that, in spite of your original, ludicrous claims, attempts to reduce CO2 emissions will *not* necessarily result in a halt to international tourism-& certainly not the very small subset that visits the Pacific Islands. So your entire argument is a *straw-man*, & none of the logical fallacies you've provided in between have made it any less of a straw-man-which is pretty typical of the card-carrying members of the Denialist Cult.

Well yes the difference by 2050 is only about 2cm/year-4cm/year but-as can be seen from where the lines end up, that *small* difference will make the difference in (a) how long we have to evacuate the more low lying atolls or build protective structures to hold the waters at bay, (b) probably protect some of the higher atolls for significantly longer & (c) decrease the amount of time it will take before sea levels actually begin to fall. Yet if we listen to people like you, we should just accept the BAU approach, & simply aim for the worst case scenario by 2100-even though the BAU approach will probably result in an even *worse* sea-level rise than what's currently predicted. Its exactly this kind of Denialist Cult thinking which has preventing action being taken earlier than now-all to protect the profits of your beloved fossil fuel industry.

Oh, & here is yet more proof that your whole "international air travel" argument is entirely bogus. A Boeing 747-400 consumes an average of 2.6L of fuel/100km/passenger (assuming at least a half-full plane). A mid-sized vehicle consumes about 10L of fuel/100km/passenger *if* the vehicle contains only the driver (as it frequently does). This can go up to 12L/100km in peak hour traffic, as cars can consume an additional 20% of their fuel simply idling. It is this kind of driving that makes cars such a massive contributor to GHG emissions, *not* the difference in the number of people using them.

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Moderator Response: (DB) Ok, this has gone far enough; future off-topic comments by all parties will be deleted.

In area of the tropical reefs just changed acidification ( here and here)

... when coral bleaching we must remember the conclusions of this work: Suggett & Smith, 2010. - notes contained therein - the recommendations:
“While this synonymous association has undoubtedly been key in raising public support, it carries unfair representation: nonlethal bleaching is, and always has been, a phenomenon that effectively occurs regularly in nature as corals acclimatize to regular periodic changes in growth environment (days, seasons etc).”
“While bleaching induced coral mortality must remain our key concern it must be better placed within the context of bleaching signs that do not result in a long-term loss of reef viability.”

Arkadiusz Semczyszak, when I saw that your first 3 links were from WUWT and CO2SCIENCE (i.e. non-scientific), I didn't read any further. You really do need to ask yourself why you rely so much (especially in that last post of yours) on second-hand information filtered through blogs.

Arkadiusz Semczyszak - See the discussion, references, and particularly Figure 3, provided in the advanced version of this rebuttal. Coral bleaching is likely to have caused the "drowning" of ancient atolls as continental plate motion transported them north through the equatorial "hot zone". And don't confuse natural bleaching with mass coral bleaching.

The first link is a figure cited by a number of websites - it’s from the pre-review paper. The next two figures originate (unchanged) also pre-reviewed the papers. I did not quote “a word” of comment to them - nor Idso or WUWT.
My information comes only from work Suggett & Smith1, 2010.. They are workers: Coral Reef Research Unit, Department of Biological Sciences, University of Essex, UK.
Instead of looking for "conspiracy theories", we respond to their proposal: that even strongly coral bleaching is a natural phenomenon, often found in colonies of corals - and often quickly reversible.
It is worth to discuss their work in detail but we can not ignore.

Arkadiusz Semczyszak- coral bleaching is a natural phenomenon, often found in colonies of corals - and often quickly reversible

No. See comment at @ 32 and note the peer-reviewed studies provided. Coral bleaching that killed ancient atolls was natural. Earth was in a "greenhouse" phase then, and the equatorial regions not conducive to coral survival. No humans around back then.

As discussed here- mass coral bleaching, a thread you have previously commented on, mass coral bleaching has only recently emerged, coincident with human-caused global warming. And note the lack of recovery of bleached coral reefs in the eastern tropical Pacific, especially the Galapagos Islands.

As for Suggett & Smith (2010). So they're upset about media reports confusing all coral bleaching events with mortality. So what?